Convergent synthesis, NMR and conformational analysis of tetra-and petasaccharide haptens of the Shigella flexneri serotype 5a O-specific polysaccharide
Résumé
Convergent syntheses of the methyl glycosides of the branched pentasaccharide -L-Rhap-(12)-[-D-Glcp(13)]--L-Rhap-(13)--L-Rhap-(13)--D-GlcNAcp [A(E)BCD], featuring the biological repeating unit of the O-specific polysaccharide of Shigella flexneri serotype 5a, and of a related linear tetrasaccharide (EBCD) are described. The strategy, based on the trichloroacetimidate methodology, relied on the use of a key EB disaccharide donor and appropriate CD acceptors. The use of an isopropylidene acetal to block OH-4 and OH-6 of residue D was found to be a suitable alternative to the employment of the more commonly used benzylidene acetal. Conformational analysis of EBCD-OMe and A(E)BCD-OMe was based on analysis of 1H and 13C chemical shifts and inter-proton distances data obtained by NMR spectroscopy. The data showed that residue A had no influence on the conformational behaviour of residue E, although these two residues were involved in a 2,3-cis vicinal glycosylation pattern in A(E)BCD-OMe. Comparison of 1H and 13C chemical shifts of the two oligosaccharides with those of their corresponding sequences in the O-specific polysaccharide of S. flexneri 5a showed that the two oligosaccharides presented a distribution of solution conformations similar to that in the O-specific polysaccharide. The conformation of A(E)BCD-OMe was investigated by two approaches: (i) energy minimisation based on ROE-derived distances with the DISCOVER program and (ii) a conformational searching method (the CICADA algorithm interfaced with MM3 force-field). The minimised conformation obtained by the former approach was in total agreement with the average of the two major families of conformations resulting from the CICADA calculations.